Containers manufactured from plastic resins are used in many industries in the packaging of goods. For instance, plastic containers are used to package automotive fluids such as motor oil, household products such as detergent, and other liquid, granular or gel products. Such containers are manufactured efficiently utilizing blow-molding techniques and can be made from various resins such as, for example, high density polyethylene (HDPE). Because of the widespread use of blow-molded plastic containers, cost saving and environmental related improvements to the manufacture of blow-molded plastic containers are of great significance to the packaging industry.
According to the Encyclopedia of Chemical Technology, third edition, volume 4, it is conventional to utilize calcium carbonate as a filler in various products to reduce material costs and improve specific qualities of the products such as heat resistance, dimensional stability, stiffness, hardness, and processability. The referenced encyclopedia suggests the use of calcium carbonate as a filler in paper, paint, plastics, rubber, textiles, putties, caulks, sealants, adhesives and printing ink.
U.S. Pat. No. 3,969,314 issued to Grigull discloses the use of fillers with plastic resins "to improve stability, to make the raw material less expensive and to achieve high heat resistance." See Column 1, lines 9-11. The patent also discloses the use of a talcum, or chalk, filler in a plastic resin to manufacture blown hollow bodies such as containers for detergent. At column 2, line 60, the Grigull patent discloses a "particularly advisable" mixing ratio for chalk and talcum fillers of between 30% to 40% filler to 70% to 60% plastic resin.
Other patents also disclose the use of calcium carbonate as a filler in plastic resins to produce various products and to accomplish various purposes. For example, see U.S. Pat. Nos. 4,526,823 and 4,626,456 issued to Farrell et al.; 4,629,596 issued to Coffman; 4,165,302 issued to Armenti et al.; 3,725,336 issued to Susuki et al.; 4,174,340 issued to Luders et al.; 3,203,921 issued to Rosenfelder; 3,976,612 issued to Kaji et al.; 3,362,924 issued to Eastman; 5,371,132 issued to Ebara et al.; 5,102,465 issued to Lamond; 4,835,195 issued to Rayfield et al.; 4,244,933 issued to Shibazaki et al.; and 4,279,661 issued to Strauch et al.
While the cost of manufacturing a plastic blow-molded container is one concern, another important concern is whether or not the container can be readily recycled. High density polyethylene (HDPE) has a specific gravity of 0.956, and therefore, floats on water for ready recovery by existing floatation tank recycling equipment. Another commonly used plastic resin for manufacturing blow-molded containers is polypropylene (PP) which has a specific gravity of 0.90 and also can be readily recycled using floatation tank equipment.
A problem with the use of calcium carbonate as a filler in plastic blow-molded containers is that such containers cannot be readily recycled using existing floatation tank recycling equipment. Calcium carbonate has a specific gravity of 1.4. Thus, a mixture of calcium carbonate filler and one of the above referenced plastic resins will have a combined specific gravity of above 1. For example, a mixture of: 85% HDPE and 15% calcium carbonate has a specific gravity of 1.004; 70% HDPE and 30% calcium carbonate has a specific gravity of 1.154; 50% HDPE and 50% calcium carbonate has a specific gravity of 1.364; 85% PP and 15% calcium carbonate has a specific gravity of 1.042; 70% PP and 30% calcium carbonate has a specific gravity of 1.126; and 50% PP and 50% calcium carbonate has a specific gravity of 1.362. Articles formed from such compositions will sink in the floatation tanks, and therefore, cannot be readily recycled utilizing existing floatation tank recycling equipment.
Although the blown container disclosed in the Grigull patent may function satisfactorily for its intended purpose, there is a need for an environmentally friendly blow-molded plastic container which is manufactured from a mixture of plastic resin and a finely divided mineral filler, such as calcium carbonate, to achieve cost efficiency. The container should be capable of being readily recycled using existing floatation tank equipment.